A liquid crystal display device is composed of a liquid crystal display panel and a drive circuit thereof, and the basic configuration of the liquid crystal display panel is such that a first substrate formed with many scanning electrodes on the inner face and a second substrate formed with many data electrodes, in such a manner to be perpendicular to the scanning electrodes, on the inner face, are bonded together with a fixed gap provided therebetween, a liquid crystal layer is sealed in the gap, and portions where the scanning electrodes and the data electrodes oppose each other with the liquid crystal layer sandwiched therebetween become pixel portions respectively.
As a driving method of this liquid crystal display panel, a method is employed which applies selection signals in a time division manner to all the scanning electrodes constituting the pixel portions of the liquid crystal display panel and applies a data signal to the data electrode in correspondence with the selection signal of each scanning electrode to induce an optical change in the liquid crystal layer at the individual pixel to thereby perform display.
In such a driving method of the liquid crystal display panel, when the number of pixels of the liquid crystal display panel is increased to improve its display quality, the time during which the signal can be applied to one pixel is decreased, and thus it is necessary to increase the voltage of the selection signal or to increase the voltage of the data signal.
Further, since the display disappears if the liquid crystal is not supplied with charge in a predetermined cycle, it is necessary to apply a predetermined voltage in a fixed cycle even for the same display contents. Therefore, an increase in the number of scanning electrodes causes an increase in the frequency for switching the voltage of the selection signal and also an increase in the frequency of the data signal.
The output voltage and the output frequency of a circuit for applying a predetermined selection signal and data signal to the liquid crystal display panel accordingly become higher to increase the power consumption of the liquid crystal display device.
When the liquid crystal display panel is used for a small portable electronic device, however, there is a limit in thickness, weight, and volume of the case thereof and there is also a restriction in battery capacity. Hence, it is required to enable operation for a long time by a battery having a capacity as small as possible.
Further, a liquid crystal display device having a power generating function is scarcely commercialized in the status quo. The reason is that the electric power consumed is very large as compared to the capacity of a storage battery which stores energy therein. Therefore, it is important to allow the liquid crystal display device to function for a time as long as possible by a predetermined battery capacity, and it can be said that it is also preferable for the earth environment.
There is a method, as a method of reducing the power consumption, which does not perform display on a part or the entire face of the liquid crystal display panel, but it is not preferable because a decrease in display area results in a reduction in the display contents.
Hence, it is desired to reduce the electric power consumed while allowing a display to be performed on the entire face of the liquid crystal display panel constituting the liquid crystal display device.
Further, in the case of a liquid crystal display device provided with a power generating element, it is necessary to balance the amount of power generated by the power generating element and the amount of power consumed by the liquid crystal display device, and therefore it is necessary to reduce the power consumption of the liquid crystal display device. Especially when a photovoltaic element is disposed as the power generating element on the observer side of the liquid crystal display panel and at a position adjacent to the liquid crystal display panel, it is necessary to decrease the area of the photovoltaic element and to increase the proportion of transmitting portions around the photovoltaic element to prevent the display quality of the liquid crystal display panel from deteriorating. Thus, it is very important to reduce the power consumption of the liquid crystal display device.
Hence, it is an object of this invention to reduce power consumption while keeping as much as possible display contents displayed on a liquid crystal display panel constituting a liquid crystal display device so as to increase battery life. Specifically, it is an object to attain a reduction in the power consumption without decreasing its display region.
Further, it is another object to substantially reduce power consumption, also in a liquid crystal display device having a power generating function, by appropriately controlling the driving waveform of a liquid crystal display panel to enable driving of the liquid crystal display panel by a power generating element with a small power generation amount which cannot be used in the prior art.